We need an Eigen 4.0 release branch

It's been over 2 years since the release of Eigen 3.4.

There has been significant improvements to the code base since then - performance, new features, general improvements and fixes, etc. Some users of Eigen are forced to use only release branches like 3.4 and not master.

I think we should start planning (or discussing) on when Eigen 4.0 can be released (especially since it is a very large release that will require things like C++14 and multiple people).

I know time and resources are limited but this issue should be a call for additional help - if needed.

Eigen is a great product - let's continue to make it so!!!